1. Field of the Invention
The present invention relates to a laser power monitor device, and more particularly, to a laser power monitor device for an optical recording and/or reproducing apparatus, capable of precisely controlling the power of a laser diode during a high speed or low speed operation of the optical recording and/or reproducing apparatus. The present invention also relates to an optical pickup including the laser power monitor device, and an optical recording and/or reproducing apparatus including the optical pickup.
2. Description of the Related Art
A laser diode is used as a light source in an optical pickup for an optical recording and/or reproducing apparatus, such as a CD player, a recordable CD-R drive, or an overwritable CD-RW drive. The power of the laser diode should be stable so that recording and/or reproducing of the optical recording and/or reproducing apparatus is performed smoothly. However, the power of the laser diode varies significantly with the operating temperature or period of use. Thus, power control should be performed in order to stabilize the power at an appropriate level. As such, a laser power control apparatus is provided to the optical pickup.
FIG. 1 shows a conventional laser power control apparatus. Reference numeral 10 denotes a photo diode 10 which receives part of the light emitted from a laser diode 40 and outputs a current proportional to an optical power of the laser. Reference numeral 20 denotes a monitor circuit, which is a current/voltage conversion circuit that receives the current output from the photo diode 10, converts the received current into a voltage, and outputs the voltage. A variable resistor for controlling the gain of the monitor circuit 20 is provided in the monitor circuit 20, and a value thereof is controlled in a process of manufacturing the optical recording and/or reproducing apparatus so that the monitor circuit 20 used in an optical recording and/or reproducing apparatus has a constant gain, that is, so that the monitor circuit 20 outputs the same voltage with respect to same current input.
Reference numeral 30 denotes an automatic laser power control circuit that receives an output voltage from the monitor circuit 20, compensates a difference between the output voltage and a predetermined reference voltage, and outputs a proper laser diode driving current.
The photo diode 10 and the monitor circuit 20 that convert power of a laser incident on the laser diode 40 into a voltage corresponding to the laser power, are referred to as a front photo detector. However, in the present specification, the photo diode 10 and the monitor circuit 20 are referred to as a laser power monitor device.
The maximum voltage that is received from the ALPC circuit 30 and can be processed is limited. As such, a voltage output from the laser power monitor device should be in the range of the maximum voltage. This should be considered when setting the gain of the monitor circuit 20.
FIG. 2 is a graph describing optical powers of a laser output from the laser diode in a pulse train system used in a CD-RW and DVD-RW recording apparatus. In a space section between pits, a laser at erase power level Pe is irradiated, and a recorded portion is erased. When recording pits in a mark section, a laser at peak power level Pp, which is a maximum power, and a laser at bias power level Pb, which is a minimum power, are alternately irradiated and the pits are recorded at fast speed. In order to properly heat dyes coated on an optical disc, an initial peak power section of the mark section is longer than the other sections. In accordance with the power input, output of the laser power monitor device ranges from the minimum voltage corresponding to the bias power level Pb to the maximum voltage corresponding to the peak power level Pp.
FIG. 2 shows part of a recording signal that indicates recording performed at a predetermined recording speed. In a recording mode of an actual optical recording and/or reproducing apparatus, a recording speed may be varied by a user's selection. Even when the recording speed is set to high speed, the optical recording and/or reproducing apparatus operates. When recording is performed at low speed, the rotational speed of an optical disc is lower than when recording is performed at high speed. In the case of high speed, recording should be performed with higher optical power. As such, the above-described power control should be performed depending on a recording speed.
FIG. 3 is a graph of laser driving current versus voltage output from a monitor circuit 20 in the conventional laser power control apparatus. When high-speed recording and low-speed recording is performed, the slope of the graph is the same, but the variation range of optical power is different. Thus, the variation range of laser driving current is different. As such, the variation range of a monitor circuit output voltage is different. In other words, in the case of low speed, the variation range of laser driving current and monitor circuit output voltage is smaller than the variation range of laser driving current and monitor circuit output voltage at high speed.
In addition, a reference voltage compared with an actual current voltage for output compensation is different for each speed. The reference voltage is low at low speed where low optical power is used. Optical power is proportional to the driving current inputted into a laser diode. Thus, similar graphs of the laser diode driving current and the monitor circuit output voltage can be obtained.
The ALPC circuit 30 tabulates and stores the relation between the laser diode driving current and the monitor circuit output voltage through a power calibration process during an initial operation of the optical recording and/or reproducing apparatus. When recording is performed, a difference between the output voltage of the monitor circuit 20 and a predetermined reference voltage is measured, and in order to compensate the difference, the laser diode driving current is increased or decreased.
The gain of the monitor circuit 20 is controlled and fixed at a constant level. Thus, in the case of high speed recording at which high optical power is outputted Pp current output from the photo diode 10 is large at the peak power level Pp. Thus, the voltage output from the monitor circuit 20 is high and has a wide range of variation. On the other hand, in the case of low speed recording at which low optical power is output, current output from the photo diode 10 is small at the peak power level Pp. Thus, the voltage output from the monitor circuit 20 is small and has a narrow range of variation.
However, during low speed recording in the conventional apparatus having the above structure, the precision of power control decreases as the variation range of the voltage output from the monitor circuit 20 and input into the ALPC circuit 30 decreases.
The maximum speed of the optical recording and/or reproducing apparatus has increased rapidly with the development of new recording and/or reproducing technologies. However, the maximum output voltage of the monitor circuit 20 is limited. Thus, as the maximum speed increases, the difference between the minimum speed and the maximum speed of the output voltage range of the monitor circuit 20 also increases. As such, lowering the precision of power control at low speed becomes more critical.
In the case of using one photo diode 10, the photo diode 10 should be large enough in order to obtain a sufficient output current. In such cases, the frequency characteristic of the output current is not good, and the variation of the output current cannot exactly follow a variation in optical power of laser incident on the photo diode 10.